<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        function backDay(){
            let year = prompt('请输入年份');
            let flag = isLeapYear(year);
            if(flag){
                console.log(`${year}年的2月又29天`);
            }else{
                console.log(`${year}年是平年，2月份有28天`)
            }
        }

        function isLeapYear(year){
            let result =false;
                  if ((year %4 ==0)&& (year % 100!=0)|| (year % 100 == 0 && year % 400 == 0)){
                    result =true;
                  }
                  return result;
        }
    </script>
</body>
</html>